Carbon is routinely added to surfaces of carbon steel components by known carburizing techniques. One of the known processes uses a carburizing atmosphere in a furnace used to heat the components to an elevated temperature to increase the carbon content of the steel surface to a controlled depth, so that the carburized piece has increased surface hardness and wear resistance after hardening. The function of a carburizing atmosphere is to provide carbon potential in the furnace and transfer carbon to the surface of carbon steel components. The carburizing process is generally carried out in a batch or continuous furnace operated at temperatures between 750.degree. C. to 950.degree. C. (1380.degree. F. to 1750.degree. F.). The rate of carburization (carbon transfer or carbon addition to the surface) depends upon the carbon potential in the furnace, the carbon potential being determined by carburizing temperature and carburizing gas composition. Although temperature plays a key role in controlling the rate of carburization, gas composition can play an important role in manipulating the rate of carburization during early stages of carburization. For example, the rate of carburization can be accelerated during early stages of carburization by increasing the partial pressure of carbon monoxide in the furnace atmosphere, as practiced in rapid carburization and accelerated carburization processes.
A wide variety of atmospheres have been employed to provide the desired carbon potential in a furnace in order to transfer carbon to the surface of carbon steel components being heated in the furnace. For example, pure methane or natural gas has been employed to carburize steel components. However, the use of methane or natural gas produces large amounts of soot in the furnace, causing the life of furnace components to be shortened. Furthermore, it is difficult, if not impossible, to carburize steel components in a controlled and consistent fashion with methane or natural gas. Therefore, pure methane or natural gas is rarely employed these days to carburize carbon steel components.
A gaseous mixture consisting primarily of 20% carbon monoxide, 40% hydrogen, and 40% nitrogen that is produced by using an endothermic generator has been employed for carburizing carbon steel components for many years. This gaseous mixture is produced by reacting mixture of a hydrocarbon gas such as natural gas or propane and air at high temperature (above about 980.degree. C., 1796.degree. F.) in a reactor packed with a nickel catalyst supported on alumina. It has, however, been difficult to maintain quality and consistency of endothermic atmosphere because of constant changes in the composition of the air entering the generator and the decrease in activity of the nickel catalyst with time. The activity of the catalyst changes with time due to deactivation by coke formation. Furthermore, because of catalyst deactivation with time by coke formation, it is not possible to use an endothermic generator to continuously produce carburizing atmosphere because the generator needs to be shut down periodically to regenerate the catalyst. Numerous advances in the design and operation of endothermic generators have been made over the years, including incorporation of an oxygen probe (or carbon probe) to improve quality and consistency of the endothermic atmosphere, but these changes have not yet solved all of the problems associated with this equipment.
Carburizing atmospheres consisting primarily of one part carbon monoxide and two parts hydrogen, produced by dissociating pure methanol, have been used for carburizing carbon steel components to overcome quality and consistency issues related to an endothermically generated atmosphere. Likewise, carburizing atmospheres consisting of 20% carbon monoxide, 40% hydrogen and 40% nitrogen produced by dissociating methanol in the presence of nitrogen, have been employed for carburizing carbon steel components. However, the use of methanol is becoming increasingly unpopular these days, due to its toxicity.
Carburizing atmospheres consisting of 20% carbon monoxide, 40% hydrogen and 40% nitrogen produced by dissociating methanol in the presence of non-cryogenically generated nitrogen have been employed for carburizing carbon steel components. The non-cryogenically generated nitrogen is produced from air by using either a pressure swing adsorption (PSA) or a membrane system. The nitrogen produced by these systems usually contain 1 to 5% residual oxygen as an impurity. The nitrogen with oxygen impurity at these levels has been used as a substitute for pure nitrogen, primarily to reduce overall cost of producing carburizing atmospheres. A small amount of natural gas is added to the atmosphere to compensate for oxygenated species such as carbon dioxide and moisture generated by using non-cryogenically generated nitrogen. Once again, as mentioned earlier, the use of methanol is becoming increasingly unpopular due to its toxicity.
A mixture of pure nitrogen, a hydrocarbon gas (methane or natural gas or propane), and carbon dioxide has been introduced directly in a batch carburizing furnace to produce a carburizing atmosphere in-situ, as disclosed and claimed in U.S. Pat. No. 4,049,472. According to patentees, carbon dioxide reacts with methane at a temperature in excess of 1500.degree. F. (815.degree. C.) inside the furnace, thereby producing the desired carburizing atmosphere. In reality, however, it has been found that the rate of thermal reaction between carbon dioxide and the hydrocarbon gas at a temperature in excess of 1500.degree. F. (815.degree. C.), is not high enough to produce the desired 20% concentration of carbon monoxide in the atmosphere. Furthermore, carbon steel components treated using the process claimed in this patent are surface decarburized rather than surface carburized. Finally, it has been found via experimentation that a temperature in excess of 1750.degree. F. (950.degree. C.) is required to produced a carburizing atmosphere in-situ from a mixture of pure nitrogen, carbon dioxide and a hydrocarbon gas. A temperature in excess of 1750.degree. F. (950.degree. C.) is generally not acceptable for carburization of carbon steel parts because of the potential of the carburized parts to have severe distortion and deformation. Therefore, the process disclosed in this patent is not suitable for producing carburizing atmospheres from a mixture of pure nitrogen, carbon dioxide and a hydrocarbon gas.
A carburizing atmosphere has been produced by substituting carbon dioxide partially or fully for air that is used to react with the hydrocarbon gas in an endothermic generator, as disclosed in German Patent DE 4343927 C1. According to this patent, the partial substitution of air with carbon dioxide produces a carburizing atmosphere with a more desirable hydrogen to carbon monoxide ratio of between 1 and 2. The complete substitution of air with carbon dioxide produces a carburizing atmosphere with 50% carbon monoxide and 50% hydrogen and a hydrogen to carbon monoxide ratio of 1. It has been unexpectedly observed that by substituting carbon dioxide completely for air in an endothermic generator used to produce a carburizing atmosphere, catalyst deactivation by coke formation occurs.
Synthesis gas consisting of a mixture of carbon monoxide and hydrogen (suitable for carburizing) can be produced by combined partial oxidation and carbon dioxide reforming of methane, as disclosed in papers by S. B. Tang et al. and A. T. Ashcroft et al. The synthesis gas or the product from combined partial oxidation and carbon dioxide reforming of methane contains a substantial amount of unreacted carbon dioxide. The combined partial oxidation and carbon dioxide reforming of methane, therefore, can be used to produce carburizing atmospheres, provided substantially all of carbon dioxide is removed from the product gas.
A number of nickel, rhodium, ruthenium, iridium, and platinum-based catalysts have been studied to produce synthesis gas or a mixture of carbon monoxide and hydrogen (suitable for carburizing) by reforming methane with carbon dioxide (dry reforming). These catalysts have been disclosed in publications by S. C. Tsang et al., J. Min et al., C. Ping et al., T Uchijima et al., K. Seshan et al., and U.S. Pat. No. 5,336,655. The processes disclosed in these papers require the use of pure methane for producing synthesis gas, which then can be used as a carburizing atmosphere. Because pure methane is very expensive, the processes disclosed in these references can not be used to produce carburizing atmosphere cost effectively.
Synthesis gas or a mixture of carbon monoxide and hydrogen (suitable for carburizing) is produced by reforming methane with carbon dioxide (dry reforming), as disclosed in UJK Patent GB 2170508A. According to the disclosed process, the formation of coke on the catalyst is mitigated by using an excessive amount of carbon dioxide. The excess carbon dioxide is then separated from the product gas to produce the desired composition of synthesis gas. Since the process requires an expensive step to remove carbon dioxide from the product gas stream, it is not economical to produce a carburizing atmosphere using this method.
Based upon the above discussion, it is clear that there is a need to develop a process for producing carburizing atmospheres with consistent quality and composition, cost effectively. Furthermore, there is a need to develop a process for producing carburizing atmospheres that (1) is not dependent on changes in the composition of air, (2) does not require expensive unit operations to remove unreacted carbon dioxide, and (3) is not prone to promote frequent catalyst deactivation by coke formation.